Solder bump interconnection techniques for both electrically contacting component packages and mounting them on interconnection substrates such as printed circuit boards have become widely used in the manufacture of electronic devices. The generic term interconnection substrates includes several forms of electronic device supports including, e.g., epoxy boards and ceramic substrates. For convenience, reference herein to such supports will be to printed circuit boards as a generic term.
State of the art component packages frequently include one or more double sided circuit boards, with the double sided circuit board solder bump bonded to another support structure, typically another printed circuit board. Contact pads are formed on both printed circuit boards in mirror arrays that mate when the boards are properly aligned together. Each of the contact surfaces on one or the other array of contact pads is provided with a body of solder in the form of a ball, or thick localized solder layer. Assembly is completed by applying heat to melt the solder and form the solder bond and interconnection between the arrays of contact pads. The solder can be in a variety of forms, e.g. bumps, balls, pads and other forms of thick layers including printed solder paste layers, and the term solder bump is used generically in this description to refer to any of these forms of solder bodies.
Double circuit boards are standard parts used in the fabrication of multi-chip module or ball grid array components. The double sided boards are provided with through hole connections typically made by drilling holes through the board and plating the interior of the hole with a conductive metal, e.g. copper. Contact areas, usually referred to as capture pads, are provided on both sides of the through hole to interconnect circuits runners on one side of the board with circuit runners on the other side. The contact areas are aligned to the through holes, or vice versa, as closely as the alignment tolerances permit but are typically significantly larger than the holes to ensure registration.
The through hole sites, consisting of the contact areas just described, are located along the edges of the printed circuit board in edge arrayed interconnection schemes, or may be located at any site on the printed circuit board in area array interconnection arrangements. In both cases the through hole sites are forbidden regions for solder bump interconnections between boards. This is due to the tendency of the solder to wick into the through hole when the solder bumps are melted during solder bump bonding. To avoid the wicking problem, solder bumps interconnections are typically offset laterally from the through hole, and a separate printed circuit interconnection is provided between the solder bump and the through hole. The interconnection efficiency is reduced in such arrangements, and the board area required for the added printed circuit features is increased.
To overcome the necessity of isolating through holes and solder bumps it has been proposed to cover one side of the through hole with, e.g. a solder mask, and apply the solder bump to the other side. This prevents wicking of the solder into the through hole, but results in a build-up of pressure in the through hole on heating of the assembly. The pressure is due to a combination of solder flux vapor and air expanding in a confined space, and may result in a large void and in extreme cases blowout of the solder bump.
A technique for avoiding wicking of solder bumps into through holes on double sided circuit boards would allow the interconnect area to be more efficiently utilized, and would allow shorter interconnections and improved electrical efficiency.